Position-measuring devices which provide an absolute position value are being used increasingly in automation technology. Certain disadvantages of what are termed incremental position-measuring devices are thereby eliminated such as, for example, the necessity of carrying out a reference run after switching on in order to find a reference position which is used as reference point for the further position measuring by counting graduation marks.
Primarily serial data interfaces are used for transmitting the absolute position values, since they make do with only a few data-transmission lines, and nevertheless, have high data-transmission rates. Particularly advantageous here are what are called synchronous serial interfaces, which have one unidirectional or bidirectional data line and one clock line. Data packets are transmitted via the data line in synchronism with a clock signal on the clock line.
For example, European Published Patent Application No. 0 171 579 describes a synchronous serial data interface having a unidirectional data line and a unidirectional clock line. Position values from a position-measuring device are read out here in synchronism with a clock signal on the clock line. Among experts, this interface is known by the name “SSI.”
On the other hand, European Published Patent Application No. 0 660 209 describes a synchronous serial interface having a bidirectional data line and a unidirectional clock line. In this case, it is possible to transmit data in both directions—from the sequential electronics to the position-measuring device and from the position-measuring device to the sequential electronics. The data are transmitted in synchronism with a clock signal on the clock line here, as well. This principle forms the basis for an interface of the Applicant known under the name “EnDat.”
In the case of both interfaces cited, the data-transmission rate is determined by the frequency of the clock signal on the clock line, that is, a reduction in the frequency of the clock signal reduces the data-transmission rate and vice versa. In this manner, adaptation to external circumstances such as the cable length between sequential electronics and the position-measuring device is possible.
An important cost factor in the connection of position-measuring devices to sequential electronics, e.g., a machine-tool control, is based on the number of connecting lines needed, since they substantially determine the price of the high-quality data cable used. In addition, the number of connector pins necessary for the plug-in connectors is thereby determined.